Inhibition Of Wnt Signaling In Chronic Lymphocytic Leukemia Has Synergistic Cytotoxicity With Ibrutinib

The Wnt pathway plays a major role in early embryonic development and in various cancers. We found that CLL cells have activated Wnt-signaling (Proc Natl Acad SciUSA 2004, PMID: 14973184) and express genes commonly associated with B-cell progenitors or stem cells, especially the Wnt factor-related genes such as LEF1, Wnt16, and Frizzled3 (FZD3). Also, compared to normal B lymphocytes, CLL cells express high levels of the putative cancer stem cell marker Aldehyde Dehydrogenase (ALDH), especially those CLL cells with high-risk prognostic features (Leuk and Lymph, 2013, PMID 22784365) and are highly sensitive to inhibitors of Wnt signaling (Proc Natl Acad SciUSA 2004, PMID: 3156152). Finally, next generation DNA sequencing studies have identified mutations in 12 different Wnt pathway genes in 15-20% of CLL patients. Collectively, the results of these studies imply that Wnt-signaling may be an excellent target for developing effective novel therapies for patients with CLL (Oncotarget 2011, PMID: 21860066).

We compared different classes of Wnt inhibitors identified by screening of natural compound and small molecule libraries for cytotoxicity in primary leukemic cells from patients with CLL. Among all the Wnt inhibitors tested, Salinomycin and Agelastatin (naturally occurring compounds targeting LRP5/6 degradation or LEF1 down-regulation as previously reported by our groups) consistently showed dose dependent cytotoxicity at concentrations lower than 1 µM. In contrast, small molecule Wnt inhibitors targeting more proximal elements of the Wnt pathway, porcupine (Wnt-C59) and tankyrase, (XAV-939) had little direct effect on in vitro CLL cell viability even at 10 µM.

The Wnt inhibitors were also utilized as tool compounds to evaluate the effect of combined inhibition of the Wnt pathway and other pathways. Interestingly, we identified synergistic activity between Wnt inhibitors and other inhibitors, including Ibrutinib and Idelalisib. By colorimetric and flow cytometry assays, we demonstrated that significant cell death was induced with combinations of the agents at concentrations at which little or no cytotoxicity was seen with either agent alone. The studies further demonstrate that the Wnt pathway is a viable target for the treatment of CLL and may potentiate other emerging CLL therapies.